Disallow union types

[Blacksmoke16]

Resolves #340

...
in spec/spec_helper.cr:16: while requiring "./spec_models"

require "./spec_models"
^

in spec/spec_models.cr:8: The type of Teacher#id cannot be a Union.  The 'primary' macro declares the type as nilable by default.

{% begin %}
^

...
in spec/spec_helper.cr:16: while requiring "./spec_models"

require "./spec_models"
^

in spec/spec_models.cr:8: The type of Teacher#age cannot be a Union.  The 'field' macro declares they type as nilable by default.  Use the 'field!' macro to declare a not nilable field.

{% begin %}